Crystal chemical studies of the 5f-series of elements. V. The crystal structure of uranium hexachloride

UCl₆ is hexagonal with three molecules per unit cell. The unit-cell dimensions and the calculated density are: a₁ = 10.95 ± 0.02 kX., a₃ = 6.03 ± 0.01 kX., = 3.56 g.cm.^-3. The space group is Cm-D_3d³ and the atomic positions are: 1 U in (0, 0, 0), 2 U in ±(, , z) with z = , and three sets of six chlorine atoms in ±(x, 2x, z) (2, , z) (x, , z) with parameters as follows: Cl_I x 0.10 z 0.25, Cl_II x 0.43 z 0.25, Cl_III x 0.77 z0.25 The structure is of molecular type. The UCl₆ molecules are octahedral with U-CI = 2.42 ± 0.10 A.